270 



Prof. J. A. Ewing. 



[June 20, 



magnetometer needle .... in such a direction as to indicate a con- 

 tinued increase of magnetisation. Precisely opposite effects followed 

 the withdrawal of the magnetising force. The settling down ox the 

 iron into a new magnetic state is thus shown to be far from instanta- 

 neous. On account of the complication caused by the free swings of 

 the needle, good observations on the drift could not be obtained with 

 this apparatus, but it was evident that whilst most of the anomalous 

 action was over in 3 or 4 seconds, the final magnetic state was not 

 attained until after about 15 or 20 seconds." Lord Rayleigh then 

 cites my observation, quoted above. 



In the following experiments Lord Rayleigh's method of the com- 

 pensating coil has been made use of for the purpose of examining 

 in some detail this "drift," or "creeping," or quasi- viscous change of 

 magnetism which follows any change in the magnetic force acting on 

 soft iron. 



The magnetometer was a light Thomson mirror directed by the 

 horizontal component of the earth's field, and having a free period of 

 double swing amounting to nearly 1J seconds. The specimen of iron 

 used in the greater number of the experiments was a straight piece of 

 thick wire 0'404 cm. in diameter and 39'6 cm. long, over which was 

 slipped a tube with a magnetising solenoid wound upon it. The wire 

 was set in a vertical position, magnetically west of the magnetometer, 

 with its top end on a level with the mirror, and generally 6 cm. 

 distant from it. The compensating coil was wound on a wooden 

 frame, which could be moved along a " geometric slide " towards or 

 from the magnetometer in the east- west line through the mirror, 

 for the purpose of balancing the magnetic effect of the iron. In 

 some of the experiments another compensating coil was used to 

 balance the effect on the magnetometer of the magnetising solenoid, 

 but generally the simpler plan was followed of including the effect 

 of the solenoid in the determination of the compensating coil's action 

 on the magnetometer. 



To prevent the vertical component of the earth's field from acting 

 on the iron, a second magnetising solenoid was wound over the first, 

 and a constant current of the proper strength to neutralise the 

 earth's field was maintained in it without interruption. The main 

 magnetising current w r as regulated by having in its circuit a box of 

 resistance coils, and also the liquid slide described in my former 

 paper.* This allowed the magnetic force to be changed either 

 suddenly or gradually, and the slide also allowed the method of de- 

 magnetising by numerous reversals of a continuously diminishing 

 magnetic force to be resorted to whenever it was desired to reduce 

 the iron to a magnetically neutral state. 



To soften the wire it was heated to redness by being slowly drawn 

 * Loc. cit., % 18, p. 537. 



